* commonalise adding a wide character so it can be reused
* make __waddbytes add wide characters if libcurses is built in wide
character mode.
* make waddch add wide characters if libcurses is built in wide character
mode.

Background characters and attributes don't need to be kept per character
cell, as they are merged when characters are added. Remove the per cell
storage and clarify the manual page. Pointed out by ruibiao@.